Artificial Muscles for Footwear Technology: Knitting Structures With Variable Elasticity

Abstract

Current commercial midsoles provide a fxed level of cushioning and elasticity regardless of gait phase or loading rate. We report a novel “artifcial muscle” midsole composite that dynamically tunes its compressive stifness and relaxation behavior by embedding nickel–titanium (Nitinol) shape‐memory alloy (SMA) wires into multilayer silicone substrates (Shore A12, A20, A30) using two hand‐knitting patterns (A and C). Specimens (undeformed height = 30 mm; frontal area = 61,213.856 mm2 or 612.139 cm2) were tested on a 50 kN Instron 5969 under displacement control. For compression (stress–strain) testing, each sample was ramped from an initial grip separation of 30 mm to a 12.5 mm gap (≈58.33% nominal compressive strain) at 200 mm/min (≈0.111 s−1 strain rate). At each voltage state (0 V⟶14 V for Pattern A; 0 V⟶10 V for Pattern C), four loading–unloading cycles were conducted at 60 mm/min (≈0.033 s−1 strain rate). For stress‐relaxation (creep) testing, specimens were ramped from 30 mm to a 12.5 mm gap (≈58.33% strain) at 60 mm/min (≈0.033 s−1 ) and then held at constant displacement for 120 s under three voltage levels (Pattern A: 0 V, 14 V, 19 V; Pattern C: 0 V, 8 V, 14 V). Under these protocols, silicone A30 with Pattern C consistently exhibited the largest actuation‐induced contraction (Δ = 27.79% in compression; Δ = 36.54% in creep/relax), while MANOVA and t-test results confrmed that substrate hardness, actuation state, and knitting pattern each signifcantly modulated compressive stress (all p <0.001). These fndings demonstrate that our SMA‐enabled midsole can switch between soft and stif modes — across ≈58.3% strain at strain rates up to ≈0.111 s−1 — ofering a tunable, durable, and cost‐efective solution for adaptive footwear.